Currently the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit (Tx) diversity in the context of High-Speed Uplink Packet Access (HSUPA). With uplink transmit diversity user equipments (UEs) that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. A fundamental idea behind uplink transmit diversity is to exploit variations in the effective channel to improve user and network performance. The term effective channel here incorporates the combined effect of far-field pattern(s) of transmitting antenna(s), the antenna pattern associated with receiving antenna(s), as well as the wireless channel between transmitting and receiving antenna(s). This is achieved by multiplying a UE output signal s(t) with a set of complex pre-coding weights wi, where i=1 . . . N with N denoting the number of transmit antennas. Pre-coding weights are often also referred to as antenna weights. The terms “pre-coding weight” and “antenna weight” are used interchangeably herein.
By using uplink transmit diversity the antenna weights used by the UE may be adapted in such a way that user and/or network performance is maximized. Depending on UE implementation the antenna weights may be associated with different constraints. Within 3GPP two classes of transmit diversity are considered:                Switched antenna diversity, where the UE at any given time-instance transmits from one of its antennas only. Thus if wi≠0, wj=0 for all j≠i.        Beam forming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beam forming it is possible to shape an overall antenna beam in the direction of a target receiver.        
While switched antenna diversity can be applied for UE implementations with a single power amplifier (PA) the beam forming solutions may require one PA for each transmit antenna.
Switched antenna diversity can be seen as a special case of beam forming where one of the antenna weights is 1 (i.e. switched on) and the antenna weight of any other antenna of the UE is 0 (i.e. switched off).
Selection of appropriate antenna weights is crucial in order to be able to exploit the variations in the effective channel constructively. Furthermore timing of changes in antenna weights is important. When the antenna weights changes the effective channel as perceived by a receiving base station, such as a Node-B, may change abruptly. For instance, in a case of switched antenna diversity with two antennas, if a UE starts transmitting from antenna 2 instead of antenna 1 then the corresponding antenna weights w1/w2 will change from 1/0 to 0/1 in an instant. As a result previous channel estimates become out of date, which may lead to worse effective signal-to-interference ratio (SIR) at the Node-B and packet errors until channel estimates have been updated and the Node-B has adjusted its channel estimates to reflect the new channel.
Irrespective of whether switched antenna diversity or beam forming is considered, the selection of antenna weights could be based on that a serving Node-B provides explicit feedback to the UE specifying the weights that should be used. This would require a feedback channel for the feedback specifying the weights. Another approach is to let the UE autonomously decide which antenna weights to apply. The selection of antenna weights could for example be based on evaluations of feedback transmitted for other purposes, such as Transmission Power Control (TPC) commands received on the feedback channel F-DPCH (Fractional Dedicated Physical Channel).
It has been decided in 3GPP, as mentioned in 3GPP Tdoc RP-090987, 3GPP Work Item Description: Uplink Tx Diversity for HSPA, to investigate uplink transmit diversity techniques for HSPA that do not require any newly standardised dynamic feedback signaling between network and UE. According to some suggested schemes the UE should monitor the TPC commands received on the feedback channel F-DPCH to select antenna weights. The F-DPCH is an already existing feedback channel and TPC commands is feedback information that is already transmitted for purposes of uplink transmit power control. For example, if a UE operating with switched antenna diversity receives a large number of consecutive TPC UP commands on F-DPCH from the serving cell, the UE may conclude that it is likely to be beneficial to switch to another antenna for transmission.
In 3GPP focus has been on uplink transmit diversity schemes where the UE autonomously select antenna weights. A consequence of this is that the Node-B neither is aware of that a change in antenna weights has occurred nor the method that the UE applies for choosing antenna weights. The schemes considered in 3GPP so far have only been based on the TPC commands, or combined TPC when the UE is in soft handover (SHO), when deciding antenna weights.
As mentioned above selection of appropriate pre-coding weights is crucial and may have an impact not only on uplink and/or downlink performance associated with the UE selecting the antenna weights, but also on uplink and/or downlink performance associated with other UEs and Node-Bs due to interference. It is therefore important to select the pre-coding weights carefully.